This invention relates in general to cable testing, and more particularly to the measurement of insertion loss or attenuation characteristics of a conductor, such as a twisted pair.
It is increasingly common for computers, workstations, printers and file servers in a modern office to be interconnected to form a local area network (LAN). These devices on the LAN are typically physically connected using twisted-pair LAN cables. An example of a twisted-pair LAN cable is an eight-wire cable which is configured as four twisted-wire pairs. These twisted-wire pairs are commonly referred to simply as “twisted pairs”. The twisted pairs are enclosed within a flexible wrapper that may include an electrostatic shield. Each end of the LAN cable is terminated in an industry-standard connector, such as an RJ-45 connector. A LAN cable that is poorly installed, or has faulty twisted-pairs or connectors can result in errors in data transmission therethrough. Therefore, during installation, the LAN cable is typically tested to verify that its parameters are within specifications before it is commissioned for use.
One of these parameters is the attenuation or insertion loss of the LAN cable for a signal having a frequency in a given frequency range. The measured insertion loss is compared with a specified insertion loss limit which indicates the maximum signal loss allowable in a given LAN cable. Attenuation of a signal when propagating through a LAN cable may be simply the result of the decrease in the power of the signal as it propagates along the cable, or the result of signal loss through a faulty twisted pair or connector. If the attenuation exceeds the specified limit, the LAN cable is considered not to be compliant with the performance specifications and will have to be replaced.
A known method of measuring insertion loss in or of a LAN cable is by use of a test instrument that includes a main (or master) unit and a remote (or slave) unit connected to a first end and a second end of the cable under test respectively. The main unit is operable to apply a swept-frequency sine wave signal at the first end of a twisted-pair of the cable. The strength of the signal is measured using the remote unit at the second end of the cable. The frequency of the signal is incremented in discrete steps across the range of frequencies. At each frequency, a signal strength measurement is taken to determine the attenuation of the test signal by the twisted-pair at the frequency. In this manner, a list of attenuation values of the twisted pair is generated. This list of attenuation values is compared with a corresponding list of worst-case attenuation values that is obtained from an attenuation versus frequency characteristic curve specified for the cable under test. Such a comparison is to determine compliance, that is, whether or not the attenuation limit according to the specification is exceeded. It is thus important to ensure that the insertion loss measurements are accurately determined.
The attenuation of the test signal (at a particular frequency) along a path that includes a transmitter TXm of the main unit, a twisted-pair and a receiver RXr of the remote unit may be represented as follows:Input−Output=LTXm(t2)+LRXr(t2)+LSection1+LSection2+LCable  (1)wherein                Input is the desired signal strength of the test signal to be transmitted by the transmitter TXm;        Output is the signal strength of the test signal measured by the receiver RXr at time t2;        LTXm(t2) is the change in signal strength of the test signal attributable to the transmitter TXm at t2;        LRXr(t2) is the change in signal strength of the test signal attributable to the receiver RXr at t2;        LSection1 and LSection2 are the insertion losses attributable to sections of the path in the main unit and remote unit respectively; and        LCable is the insertion loss of the twisted-pair of the cable under test.        
In order to determine LCable, the same measurement is performed for a corresponding twisted-pair of a calibration cable having a known insertion loss. The attenuation of the test signal along a similar path may be represented as follows:Input−OutputCable—calib=LTXm(t1)+LRXr(t1)+LSection1+LSection2+LCable—calib  (2)wherein                OutputCable—calib is the measured signal strength of the test signal received by the receiver RXr at a time t1;        LTXm(t1) is the change in signal strength of the test signal due to the transmitter TXm at t1;        LRXr(t1) is the change in signal strength of the test signal due to the receiver RXr at t1; and        LCable—calib is the insertion loss of the twisted-pair of the calibration cable.        
The insertion loss, LCable, of the twisted pair of the cable under test may be determined by combining equations (1) and (2) as follows:Output−OutputCable—calib=LTXm(t1)−LTXm(t2)+LRXr(t1)−LRXr(t2)+LCable—calib−LCable  (3)
The above equation (3) may be rewritten as follows:Output−OutputCable—calib=ΔLTXm+ΔLRXr+LCable—calib−LCableLCable=ΔLTXm+ΔLRXr+LCablecalib−Output+OutputCable—calib  (4)wherein                ΔLTXm is the difference in test signal strength change attributable to the transmitter TXm at time t1 and t2; and        ΔLRXr is the difference in test signal strength change attributable to the receiver RXr at time t1 and t2.        
From equation (4), it can be seen that the insertion loss of the cable is affected by any difference in test signal strength change attributable to the transmitter TXm and the receiver RXr at the time the calibration is carried out, t1, and the time of measurement of the twisted-pair of the cable under test, t2. For the insertion loss of the cable to be accurate, ΔLTXm and ΔLRXr should ideally be zero. Unfortunately, such is not the case. The transmitter TXm and the receiver RXr include active components whose operating characteristics are temperature dependent. Thus, if the two measurements are carried out when the temperature of the test instrument is changing, such as during a warming-up period, ΔLTXm and ΔLRXr will have non-zero values depending on the difference in temperature at time t1 and t2. These temperature dependent non-zero values cause the insertion loss at each signal frequency to vary or drift to result in discrepancies in the insertion loss measurement. Consequently, the insertion loss measurement may not be accurate or repeatable.
A twisted pair, which rightfully is marginally within the specified limit according to a LAN cabling standard, such as the ISO/IEC, the TIA/EIA, the Cenelac LAN cabling standards, may thus be deemed to exceed the specified limit because of the contributions of ΔLTXm and ΔLRXr.
One way of obtaining the insertion loss of a cable under test that is less dependent on temperature of the test instrument is to perform calibration of the test instrument only after the temperature of the test instrument has stabilized. Alternatively, calibration may be performed immediately before each measurement when the test instrument is used during the warming-up period of the test instrument. However, since the temperature may take as long as half an hour or more to stabilize within the test instrument, such solutions are not very practical.